This proposal describes a concerted set of experiments directed towards a detailed exploration of the structures of fibrinogen and fibrin, information upon which any genuine understanding of clot formation and destruction must rest. The plan calls for the isolation of various domains and subdomains of the human fibrinogen molecule, including the "coiled coil" interdomainal connectors, the alpha-chain protuberances, and various derivative forms of fragments D and E. We also plan to synthesize two disulfide-containing structures corresponding to key parts of the molecule whose structures will remain equivocal until synthetic isomers are made and compared. Another undertaking involves the affinity-labeling of the two different polymerization sites located on the terminal domains. The proposal also provides for the identification and characterization of variant human fibrinogens that exhibit defective polymerization, particular attention being paid to those variants that show diminished binding of certain synthetic peptides involving the sequences Gly-Pro-Arg and Gly-His-Arg that simulate key donor binding sites. A method for purifying the variant molecules from heterozygous mixtures is described and a protocol laid out for identifying and characterizing variant peptides from these fibrinogens. Finally, the project provides for the completion of the amino acid sequence of a fibrinogen from a primitive vertebrate (the lamprey) in order that the structurally most critical parts of the molecule may be identified; there is good evidence on the basis of other molecular systems that the important parts of proteins are conserved during evolution whereas less critical features may change radically. The information derived from all these experiments bears heavily on the phenomena of fibrin formation, stabilization and fibrinolysis, all of which have profound clinical reflections.